Directional monitoring system



Sept. 30, 1941.

l B. L. BuxroN DIRECTIONA'L MONITORING SYSTEM Filed July 9, 1940 .eZEz

INVENTOR A v cmco/a/ MMM ATTORNEY Sept. 30, lui-)41.y 5.1L.;A BuxToN 2,257,592

DIRECTIONAL MONITORING SYSTEM iNvENfoR ATTORNEY Patented Sept. 30, 1941 DIRECTIONAL MONITORING SYSTEM Bentley Leo Buxton, Elmhurst, Ill., assignor to American Telephone and Telegraph Company, a corporation of New York Application July 9, 1940, Serial No. 344,503

9 Claims.

This invention relates to telephone circuits and systems and particularly to monitoring arrangements for telephone circuits or systems. More particularly, this invention relates t'o monitoring arrangements which provide means for equalizing the volume of the signals received by the monitoring set from the circuits transmitting signals to the monitoring set.

A.monitoi1ing set is often-used in the telephone plant to observe transmission for example, between two subscribers. Where the two subscribers are at widely diierent distances from the monitoring set, the observer at the monitoring seti will receive the nearby subscribers signals at a lsubstantially greater volume than the signals of the ldistant subscriber. VBecause of the proximityl of the nearby subscribers set to the monitoring` apparatus, the observer will also receive room noises audible at. the nearby subscribers set. from the two'subscribers, with the accompanying noise, is a source of annoyance to the monitoring observer and, moreover, the monitoring 'operation' is rendered diilcult by these condi- Onel of the objects of z this invention is to provide a vmonitoring set in which the levels of received signals from two subscribers, forexample, will be substantially the same.

Another of the objects of this inventionis to i substantially reduce room noises as received by the monitoring set from the transmitter of the nearby subscribers or operator's set.

These and other objects of this invention will be better understood from the detailed description hereinafterfollowing when read, in connection with the accompanying'drawings in which Figure 1 shows one formof circuit arrangement for practicing the invention, Fig. 2 illustrates a schematic of the circuit arrangement of Fig. l, Figs-.3' and/l illustrate circuits usedfor explaining the operation of the system, and Fig; 5 illustra-tes another form of circuit showing some of the switching details ofthe invention.

Fig. 1 of the drawings illustrates-two. typical telephone subscriber sets S1 and S2 each of which may include a telephonetransmitter T, a telephone receiver R1, an induction coil or transformer W1 coupling the circuits of the transmitter T and receiver R1 and a condenser C1 conf The diierence in the levels of the signals nected across some of the turns of one of the windings of the induction coil or transformer W1. The usual loop circuits that may be associated with these subscribers sets are designated L1 and L2, respectively. The loop circuit L1 may be connected through a repeating coil or transformer W2 to a local office, for example, the local office at which the `directional monitoring circuit D is located. The loop circuit L2 may be connected to the same local cnice through a similar repeating coil or transformer W3 and through a trunk circuit or intermediate oiice L3 (or series of trunk circuits or intermediate oflices). The trunk circuit (or circuits) La is generally required when the distance between one of the subscribers sets, such as the one designated S2, is outside the area served by' the local ofnce of another subscribers set to which speech is tobe transmitted in the usual way.

By interconnecting the repeating coil W2 and the trunk circuit La, the two Subscribers having the subsets S1 and Sz may converse with each other in the usual way. An operator may listen in on the conversation by connecting an operators set orl a telephone receiver across the interconnecting circuit as is vwell known. However, the operator will usually find that the'volume of the speech signals received from the subset S1 will be substantially greater than the volume of the signals received from the subset S2 and that the greater the distance between the subscribers set S2 and the operators set the greater will be the margin between the two signaling currents. If the latter distance is great enough, the speech currents coming from set S2 may become only fairly audible or even entirely inaudible; Furthermore, room noise, etc., emanating from the subscribers station S1 will be quite audible and alone may overpower the currents arriving from set S2. Such conditions make supervision difficult and ofttimes impossible. They also sometimes result in reports of service failures when there are in fact no such failures. If an amplifier were interposed between the operators set and the interconnecting line, the low volume signals of the distanct set could be increased in amplitude to any desired level, but the signals coming from the nearby telephone set would also be increased in amplitude by the same amount and the extraordinarily high level of the latter signals together with the noise currents would still be intolerable. The present invention has been designed to overcome the effects of such conditions by the use of a socalled directional monitoring circuit which will now be described.

The directional monitoring circuit comprises two repeating coils or transformers W4 and W5, a receiver R2 and a balancing network comprising two resistors Y1 and Y2 and a condenser C:

connected in series with said resistors, together with another condenser C3 connected across the resistor Y2. The transformers W4 and W5 may be of similar construction, each including two windings having some suitable impedance ratio as, for example, about 40 to 1. The low impedance winding of the transformer W1 is connected to one of the windings of the transformer W2, as shown, while the high impedance winding of this transformer is bridged by the balancing network above referred to. The low impedance winding of the transformer W5 is connected in series both with the receiver R2 and the balancing network, as shown, while the high impedance winding of the latter transformer is connected to the low impedance winding of transformer W4 and to one of the windings of transformer W2.

Fig 2 illustrates a schematic of the circuit arrangement shown in Fig. 1. I-Iere the reference characters E1 and E2 represent the voltages generated at the transmitters of the subscribers sets S1 and S2, respectively. Z1 represents the impedance of the short line, that is, the loop L1 together with the set S1, and Z2 likewise represents the impedance of the long line, that is, the trunk circuit L3 and the loop L2 together with the set S2. The low and high impedance windings of the transformers W4 and W5 of Fig. 1 are designated XA and XB, respectively, in Fig. 2. 'Ihe impedance of the observers receiver is designated Z3 an-d the balancing network Z4.

It will be clear that when currents are transmitted from the set S1 of Fig. 1 to the set S2 and none are transmitted in the opposite direction from set S2, the eifect of generator E2 of Fig. 2 may be neglected. Similarly, when currents originate at set S2 of Fig. l and none at set S1, ,the effect of generator E1 may be neglected.

When currents are generated by both sources of electrom-otive force E1 and Ez, currents designated I1, I2, I3 and I4 Will floW through the four meshes indicated in Fig. 2. Four equations may be written for the voltages in these meshes, each equation representing the voltage conditions resulting from the now of the four currents I1-I4. These equations are:

in which A1 and Az are minors of the values indidated in the expression at the right of the sign of equality and A (del) is the usual determinant denominator.

The currents I2, I3 and I4 may be obtained from similar determinants, the general expressions of which are given as follows:

where the coefficients of E1 and Ez are corresponding minors of the determinants.

When transmitting current from the nearby transmitter T of set S1 to the distant telephone set S2, E2=0, and the ratio of the power, eX- pressed in db., available in the distant receiver to that at the receiver R2 of the monitoring set D is as follows:A

Likewise, when the distant set S2 is transmitting current to the set S1, the ratio of the power, expressed in db., available at the nearby receiver to that at the receiver Rz is as follows:

P 1` Z1 l0 log ID3-20 log Z3 The foregoing equations generally define the transmission performance of the arrangement of this invention. The theoretical factors of these equations are supported by experimental proofs obtained from measurements made with circuits actually constructed in accordance with the invention.

The powerj loss Equation 9 can also be expressed as follows:

l0 log ggg-:20 log +10 log In deriving Equation 11 second order terms have been neglected. In the repeating coils or trans- In these equations XA and XB are the impedances of the windings labeled XA and XB, respectively, in Fig. 2, and XM is the mutual impedance between these two windings.

The current I1 flowing through the nearby subscribers set is given by the following determinant obtained from the simultaneous Equations 1 to 4:

formers W4 and W5, which were ofthe Western Electric B4G type, the ratio XB is small and hence computations based upon Formula 11 are sufliciently accurate for practical purposes.

From Equation 11 it can'be observedthat when the impedance of the distant .station equals Ithe impedance tof 4:the balancing network, .i. ,e.,'when .'Z2=Z4, Vthe denominator .of the second .term of the equation becomes Zero and the loss becomes very large (theoretically infinite). .If Z4=0, then Equation 411 maybe resolved into y.the gfollowing equation:

The loss expressed Yby Equation 12 is that which would .be expected if but one repeating .coil 4or vtransformer were employed for monitoring purposes as is the usual practice.

YThus it will be observed that a large loss will be obtained when Z2=Z4. When Z2Z4, the discrimination will depend on the vrelative magnitudes of Z2 and Z4. Hence if additional means were provided for changing .the effective values of either of theseimpedances, the amount of dis- -crimination can be adjusted toy any desired en 23+@ e a l logP3-2O logZl+Z4+lO log XA Z3 (13) If Z1=Za, .Equation i3 will be reduced to --that for a single repeating coil bridged across the line and the .loss will be independent of the value of Z4. If Z1.Z3, however, then .the loss will bechanged and wilLdepend on the difference in the magnitude of these two impedances.

The foregoing equations give the current, voltage and powerconditions of the system involved in this invention. The operation of the system may now .perhaps vbe better visualized from the circuit of Fig. 3.

Assume that the transformers W4 and W5 `are equal in all respects and that they have a turn ratio of 1 to Vf. If the circuit-extending between the receiver impedance Z3 and the .balancing network impedance Z4 is opened as at a. and b, then the source of voltage of the nearby-set having impedance Z1 will cause a ilow of current i1 in the circuit, as indicated Vin the drawings. 'I'he voltage :drop E1 will divide between two impedances of magnitudes Z2 and Z2/'4O, in proportion to these magnitudes. The voltage across impedanceZ4 with the circuit still open. atja fand b will be vsince \/74 0l is the turn ratio of the transformer. The voltage across Z4 willhence equal Furthermore, the voltage across impedance-Z2 will be 1'1Zz. Also, the voltage induced in the smaller winding of ktransformer W5 willbe -If the contacts a and 1b are then..closed,two

.equal andopposite voltages of magnitudes will besimultaneously impressed on the .receiver impedanceZa and vtherefore no currentwill flow throughthe receiver element. In other words, the current obtained 'by actuating .the trans- .mitter ofthe nearby subset will produce practically no-current in the receiver elementZs.

When the distant'transmitteris operatingand applying a voltage E2, the circuit conditions '.Will {be-somewhat as shown in Fig. 4. Here'thedistant voltageEz generates a current i2 in the circuit of impedance Z1. This current williinduce a voltage through transformer W4 which 'iseffec- `tive .across thenetworkZ4 and is cf a magnitude This current will also induce a-voltage l1 L' Z -g- (l2 b2 QW/Llo which will appear across the V.terminals :of vthe .low impedance lwinding of transformer `W5. Hence, upon closure of contact a, a-voltage .will be impressednpon the receiver-.element ,Z3 which is of a magnitude a value which is the difference ofL-the :tv/.ocoact- -ing voltages.

Thus the distant transmitter will cause .afsubstantial voltage'to be impressed-upon the'receiver element Z3. Thesystem therefore -discriminates .against-.the transmission from the nearbyset.

Fig. 5 illustrates an Uarrangement -for A.observing one or more subscribers circuits :as v,in the No. :4 service observing desk set wellknownin .the art. The line extending to one of the subscribers'is designated L4 and L5 and the line extending to a PBX ora central oiiice isdesignatedfLs andLs. rEhelatter line may be connectedto another Isubscribersset as will .be clearly understood.

Byinserting the .plug P intoj-ack J, `the operator maybe .apprisedof the fact that the subscriber connected to the line Lfi-Lshas removed his receiver from its hook. Upon the removalof the receiver of this line from its hook, directeurrent will ow over a circuit extending `from conductor L4 through the tip contacts of Yjack J and plug P, then .throughlcontacts Il and.l2,then through the winding of relay vD1 .and over the sleeve contacts o'f plug Pand jack-J and :then over conductor Le. In other words, the line L4-L5 will then be connected to line L15- Ls through the winding or krelayD1 and the ,lamp M1 Vwill be illuminated yto indicate .to the operator that the circuit to be observed hasbeen completed and is ready for observation.

When the lamp M1 .becomes illuminated, the operator may `loe connected to the ,subscriberfs circuit for observing the circuitand .thisis accomplished merely by operating .the key K. 'Upon the operation of key K, the conductor -L4 '(of line L4L5) will be connected .to conductor Ls (of line 'L5-Le) through the -low impedance winding of transformerWr. .The interconnecting circuit includes conductor L4,vthe.tip contacts of jack J and plug P, contact |3 and armatureM of key K, the low impedance winding of transformer W4, armature I5 and contact i6 .ofkey K, -the winding of relay D1, the sleeve contacts of AplugP and jack J and conductor La. It will be low impedance winding of transformer W4. Also, the balancing network Y1, Y2, C2 and C3 is connected to the high impedance winding of transformer W4.

The operation of key K also connects the high impedance winding of transformer W across the lines Li-Ls and L15-L6. This circuit will include conductor L6, the sleeve contacts of jack J and plug P, the .winding of relay D1, contacts I2 and l'of key K, armature I5 of key K, the high `impedance winding of transformer W5, armature Il and contact I8 of key K, condenser C4, the ring contacts of'plug P and jack J, contact I9 and armature 2li of jack J and conductor L5. The line L4-L5 will have the low impedance winding of transformer W5 in series therewith.

operator may then operate key K whenever` desired to listen in on the circuit. The circuit provided by operating key K fully corresponds to the circuit of Fig. 2.

The rheostat Z6 is first adjusted to a low and practically negligible value by the listening operator when operating key K. This will reduce the click heard by the subscriber. The operators receiver is connected to the observing circuit through an amplier A. This amplifier A is optional to the system. The rheostat Z6 may then be adjusted to some suitable value to provide proper discrimination between the two speech volumes received at R2.

The conductors 3i), 3| and 32 may be wired to other cord circuits to be monitored as is the practice with the No. 4 service observing desk set. One key, such as K, and its associated wiring would be provided for each monitoring position as will be understood by those skilled in the art.

TheA plug and jack arrangements shown in Fig. 'may be replaced, if desired, by plugs of the usual types at the main frame which are inserted in place of heat coils. It will be understood that relays may be used in place of keys of thevkind shown in Fig. 5 to provide suitable connections to the particular lines to be monitored.

The monitoring set disclosed may not only be utilized to equalize the volumes of the signals from two subscribers connected to the monitoring set, but it is equally useful to equalize the volumes of signals received from two operators, or from an operator and subscriber, or any tWo such sources whatever.

The arrangement disclosed herein may be balanc-ed to ground against longitudinal currents if so desired. This could bedone by so arranging transformer W4, for example, that one-half of its low impedance winding will be in series with one side of the interconnected line and the other half in series with the other side of the line.

While this invention has been shown and described in certain particular arrangements merely for the purpose of illustration, it will be under- :observed that a rheostat Z6 is bridged across the stood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit 0f the invention and the scope cf the appended claims.y

What is claimed is: f

1. Apparatus for monitoring a circuit interconnecting two telephone substation sets, comprising two transformers each having low and high impedance windings, the low impedance winding of one of the transformers being connected in series with the two telephone substation sets and the high impedance winding of the other transformer being connected in shunt across the interconnecting circuit, a' balancing network connected across the high impedance winding of the first-mentioned transformer, the operators receiver being connected in series with the low impedance winding of the second-mentioned transformer and the balancing network.

2. A systemfor monitoring a telephone circuit so that the volumes of the signals transmitted in Opposite directions over said Vtelephone circuit will be substantially euualized, comprising a coil connected in series with the telephone circuit, a second coil connected in shunt across the telephone circuit, abalancing network bridged across the series coil, and a monitoring receiver interconnecting the shunt coil and the balancing network.

3. A directional monitoring system for a circuit interconnecting two telephone sets, comprising a first transformer, one of the windings of which is connected in series with the interconnecting circuit, a second transformer, one of the windings of which is connected in shunt across the interconnecting circuit, a balancing network bridging the other winding of the rstmentioned transformer, and a receiver interconnecting the other winding of the second transformer and the balancing network.

4. Apparatus for monitoring a circuit interconnecting two telephone sets for equalizing the volume of the signals received from the two sets,

' comprising first and second transformers, one

of the windings of the first transformer and one of the windings of the second transformer being connected in series and in shunt, respectively, with the interconnecting circuit, a balancing network bridging the other winding of the first transformer, and a receiver interconnecting the other winding of the transformer with the balancing network, the balancing network including elements of capacitance and resistance.

5. The combination of two telephone substation sets, a circuit interconnecting said two sets, rst and second transformers, one of the windings of the first transformer being in series in the interconnecting circuit, a rheostat bridging said series winding of the first transformer, a balancing network bridging the other winding of the first transformer, one ofthe windings of the second transformer bridging the interconnecting circuit, a. monitoring receiver, the other winding of the second transformer being connected to the balancing network through the monitoring receiver, the balancing network including a plurality of impedance elements.

6. The combination of two telephone sets, a first lcoil connected in series with the two sets, a rheostat bridging said coil, a second coil connected in shunt across the circuit interconnecting said sets, a third coil coupled tothe first coil, a receiver, a fourth coil coupled to the second coil and connected through the receiver to the third coil, and an impedance network shunting the third coil.

'7. The combination of two telephone sets, a long circuit interconnecting said sets, means for monitoring the signals transmitted over the circuit, said monitoring means comprising first and second transformers, one of the windings of the first transformer and one of the windings of the second transformer being connected, respective- 1y, in series and in shunt with the interconnecting circuit, a balancing network shunted across the second winding of the first transformer, and a monitoring receiver interconnecting the other winding of the second transformer with the balancing network.

8. Apparatus for monitoring a long circuit interconnecting two telephone sets at any point along the circuit, comprising rst and second transformers, one of the windings of the rst transformer and one of the windings of the second transformer being connected, respectively, in

series and in shunt across the long circuit, a balancing network, 4a rheostat, said balancing network and said rheostat being independently connected across the rst winding of the first transformer, an ampliiier, and a receiver connected to the output circuit of the ampliiier, the input circuit of the amplifier interconnecting the second winding of the second transformer with the balancing network.

9. A system for monitoring a long telephone circuit at any point along the circuit comprising a rst coil connected in series with the telephone circuit, a second coil connected in shunt across the telephone circuit, a balancing network connected in shunt across the rst coil, a monitoring receiver, an amplifier the input circuit of which interconnects the second coil with the balancing network, and a monitoring receiver connected to the output circuit of the amplier.

BENTLEY LEO BUXTON. 

